The present invention relates to an image-forming method wherein dye images are formed on a printing sheet by a thermal transfer system. The present invention further relates to a thermal transfer system comprising, in combination, an ink ribbon and a printing sheet for conducting the image-forming method.
Efforts have been made to form images by a thermal transfer system, wherein images that are taken by electronic still cameras are printed out on printing sheets. Silver salt photographs are representative of this process.
In a thermal transfer system, an ink ribbon containing a dye is contacted with a printing sheet having an image-receiving layer thereon. The ink ribbon is heated, for example, by a thermal head, resulting in the dye in the ink ribbon to transfer onto the image-receiving layer of the printing sheet. A polyester resin is used as the image-receiving layer of the printing sheet, and the dye of the ink ribbon that is used is a disperse dye.
As disclosed in Japanese Laid-open Patent Application Nos. 1-259989 and 1-275096, various modifications have been made to the disperse dye that is used in the aforementioned system. However, once transferred to the image-receiving layer, the dye is attached to the layer only by interactions with the polymer of the image-receiving layer, for example, Van der Waals force, dipolar force, hydrogen bonds, and the like. Therefore, after formation of the images, if the dye is contacted with a resin or a solvent which has a greater affinity for the dye or, if a thermal energy is supplied to the extent sufficient to offset the interaction, migration or dissolution of the dye is induced, resulting in the images being blurred.
To overcome these disadvantages, image formation through chemical bonds has been proposed utilizing several methods as disclosed in Japanese Laid-open Patent Application Nos. 59-78893, 60-2398, 60-110494, 60-220785, 60-260381 and 60-260391.
In one method, a dye is used which has a group that is reactive with an epoxy group or isocyanate group, and an image-receiving layer which contains a compound having an epoxy or isocyanate group. In another method, a dye is used which has an acryloyl group or methacryloyl group, and an image-receiving layer which contains a compound having active hydrogen. In yet another method, a dye is used that is capable of forming a metal complex, and an image-receiving layer which contains a metal compound. Also proposed is a method wherein a dye is formed by sublimating a low molecular weight compound having an active methyl or methylene group for reaction with an aldehyde or nitroso compound in an image-receiving layer.
These methods that utilize the chemical bonds, however, have several disadvantages. First, the reactivities of the dyes and the image-receiving layers are so high that storage properties are not suitable. Further, the reaction is not completed within a short time, thus requiring an undesirably long time for the formation of images. It is also difficult to prepare dyes, and the types of usable dyes are limited. In addition, fixation is not always satisfactory.